Personal air sampling system and pump for use therein

ABSTRACT

An improved personal sampling pump design makes use of non-circular diaphragms within the pump assembly to provide a compact, space saving design. Both upstream and downstream dampers are employed to reduce pulsation in the airflow. Flow control is achieved by monitoring the back pressure created by a flow restriction or adjustable valve located at the sampler&#39;s outlet. The created pressure is monitored by a pressure sensor and is directly related to inlet flow rate. The use of an adjustable restriction on the outlet provides increased dynamic range of the flow measurement system without excessive loading to the pump.  
     A signal conditioning circuit takes the signal from the pressure sensor and feeds it to an analogue to digital converter which in turn feeds it to a microprocessor. The microprocessor controls the pump drive circuitry such that a constant inlet flow rate is maintained.

[0001] This invention relates to a personal air sampling system, and a pump for use in such a system. The pump may also find other applications in which a low-cost device is required to move relatively small volumes of air, for example in medical or laboratory applications.

[0002] Personal air sampling systems typically are carried by workers exposed to dusty or otherwise contaminated atmospheres. In such a system a pump develops a partial vacuum to pull air through a filter or other device which collects airborne particles or otherwise traps or senses a contaminant in the atmosphere. Provided that the volumetric flow rate of air through the device is known, operation of the system for a defined period of time yields information on the concentration of contaminant in the atmosphere to which the worker has been exposed.

[0003] However, with the passage of time, particulate material collects on the filter and increases its resistance to flow, with the result that the flow rate decreases unless the power input to the pump is increased. A known method of doing this (Peck U.S. Pat. No. 5,107,713) makes use of an empirical relationship between motor speed and flow rate. Another (Betsill U.S. Pat. No. 5,163,818) maintains a constant motor speed by varying the motor voltage. Both of these methods however depend on the very indirect indication of flow rate afforded by motor speed. The present invention in one aspect seeks to provide a method of control more closely related to actual flow conditions.

[0004] In one aspect the invention provides an air sampling system comprising a pump for drawing air through a sampling device, a local flow restriction downstream of the pump, a transducer for measuring a pressure drop across the restriction and for producing a signal indicative thereof, and control means containing a correlation between the pressure drop and the volumetric flow rate of air through the restriction, the control means being configured to control the pump to produce a said pressure drop corresponding to a desired flow rate.

[0005] The local flow restriction may be an orifice or a porous plug eg. of a gauze material. Alternatively, the local flow restriction may be adjustable; it can for example be a needle valve. An adjustable restriction allows the pressure generated by the pump (and hence its power consumption) to be minimised for a given flow rate: typically, personal sampling systems are set to an exact flow rate before use, eg. by calibration using a reference flow meter or a bubble meter.

[0006] A pulsation damper (eg. a cavity with a wall formed of a flexible membrane) may be provided upstream and/or downstream of the pump.

[0007] The system may include a temperature sensor, for sensing the temperature of the air passing through the restriction, the control means adjusting the said correlation according to the sensed temperature.

[0008] The temperature sensor may be located anywhere in or close to the airflow through system, but conveniently may be in the downstream pulsation damper.

[0009] As a personal sampling pump is worn by the user, size and weight are important. Also, the efficiency of the pump (how much electrical power is required to drive the motor in order to produce a given flow) is important because the sampling pump is powered by batteries and if the pump is more efficient then the same run-time may be achieved using smaller, and hence lighter, batteries.

[0010] Further aspects of the invention seek at least in preferred embodiments to provide a pump in which high output is combined with compact dimensions.

[0011] In a second aspect, the invention provides a displacement pump for an air sampling system comprising a pumping chamber having a displaceable wall, a motor, and drive means driven by the motor for reciprocating the wall, the wall having a major dimension and a minor dimension, and being disposed with its major dimension substantially parallel to a major dimension of the motor.

[0012] The major dimension of the motor may be parallel to an axis of rotation thereof.

[0013] There may be two pumping chambers, each with a displaceable wall disposed on opposite sides of the motor.

[0014] The drive means may comprise an eccentric mounted on a shaft of the motor.

[0015] Each wall may have semicircular ends joined by straight sides.

[0016] The or each wall preferably may be a diaphragm.

[0017] Thus, in a third aspect the invention provides a displacement pump comprising a pumping chamber having a displaceable diaphragm, a motor and drive means driven by the motor for reciprocating the diaphragm, the diaphragm having a major dimension and a minor dimension, and being disposed with its major dimension substantially parallel to a major dimension of the motor.

[0018] The or each diaphragm may comprise a membrane connected around its edge to a wall defining the remainder of the chamber by a roll section. This can enable the entire membrane to be moved bodily through a distance equal to the reciprocating stroke of the drive means thereby maximising the volume of air displaced by each stroke.

[0019] The invention now will be described merely by way of example with reference to the accompanying drawings, wherein:

[0020]FIGS. 1A and 1B are longitudinal sections through two pumps according to the invention;

[0021]FIG. 2 is a cross-section on line A-A through the pump of FIG. 1B;

[0022]FIGS. 3A and 3B show a diaphragm of the pump of FIG. 1B, FIG. 3B being a section on line A-A of FIG. 3A; and

[0023]FIG. 4 shows a personal air sampling system according to the invention.

[0024] Referring to FIGS. 1A, 1B and FIG. 2, a pump according to the invention comprises a generally cuboid body 10 defining one (FIG. 1A) or two (FIG. 1B) pumping chambers 12. Each pumping chamber is defined by walls 14 of fixed structure and a displaceable wall 16 formed by a diaphragm having a peripheral semicircular section roll 18 the outer edge of which is anchored between outer 20 and middle portion 22 of the fixed structure.

[0025] The flxed wall 14 of the chamber includes outlet 24 and inlet 26 non-return valves, formed of discs which are lightly sprung-biased against respective valve seats. The inlet valve(s) communicate(s) with an inlet plenum chamber 30, and the outlet valve(s) communicate with an outlet plenum chamber 28. These chambers in turn communicate with inlet and outlet ports of the pump (not shown).

[0026] Attached to the centre of the diaphragm 18 by upper and lower plates 33 of the same shape as but smaller than the diaphragm is a yoke or connecting rod 32 having a laterally extending slot 34. In this slot is disposed an circular cam 36 eccentrically mounted on the shaft 37 of a motor 38 which is mounted within the body 10. Operation of the motor 38 rotates the eccentric 36 which reciprocates the yoke 32 vertically with a stroke twice the offset of the centre of the cam 36 relative to the motor shaft.

[0027] The yoke thereby displaces the or each diaphragm 16 relative to its chamber in the manner of a piston, the roll section 18 permitting the central portion of the diaphragm to move as a whole relative to its fixed edge. The cyclic change in volume of the chamber 12 thus is maximised for each stroke. Enlargement of the chamber 12 causes the inlet valve 26 to open and admit air. Upon reversal of the diaphragm movement the inlet valve closes and the pressure in the chamber opens the outlet valve 24 permitting the air in the chamber to be expelled.

[0028] Referring to FIGS. 3A and 3B the diaphragm 16 is a single piece of elastomeric material such as silicone rubber having around its edge an approximately semicircular roll section 18 and a peripheral lip 40 which is received in a corresponding groove in the portions 20, 22 of the pump body. The diaphragm thus is firmly clamped around its edge in an air tight manner. A central hole 42 permits a fixing eg. a screw to pass through the diaphragm and clamp the diaphragm between the plates 33 to the yoke 32. Alternatively two fixing holes spaced along the major axis 48 of the diaphragm may be provided.

[0029] The diaphragm is generally oval in shape, having semicircular ends 44 joined by straight sides 46. The diaphragm is fixed to the yoke 32 with its major (longer) axis 48 parallel to the shaft of the motor 38. The minor axis 50 of the diaphragm is not significantly greater in length than the diameter of the motor 38. Thus the diaphragm does not project sideways materially beyond the motor, and the thickness of the casing 10 is kept within reasonable limits.

[0030] A conventional diaphragm would be circular in shape, with the result that if it were of diameter equal to the minor axis 50 of the FIG. 3 diaphragm it would be of only small area, and the pump output would be limited. Conversely, if it were circular of diameter equal to the major axis 48, the pump output would be greater but the thickness of the casing would be undesirably increased.

[0031]

[0032] The diaphragm of FIG. 3 provides an advantageous compromise: by having an extended major axis 48, a usefully larger diaphragm area is achieved without an increase in the thickness of the casing 10.

[0033] The personal air sampling system of FIG. 4 comprises a sampling head provided with a filter 56. Air is drawn through the filter by a pump 58 so that airborne particulate matter is captured on the filter for subsequent measurement and analysis. The pump 58 preferably is (but need not be) as previously described. A pulsation damper 60, in the form of a vessel or cavity with a flexible elastomeric wall, smooths fluctuations in the partial vacuum applied to the filter 56 by the pump 58. Without this damper, variations in air velocity through the filter may cause particles to be dislodged from the filter material. Other types of sampling head, eg. a size-selective device such as a cyclone, may be subject to increased error if operated in an air flow which is not smoothed by a pulsation damper.

[0034] A further pulsation damper 62 smooths pressure pulses in the flow of air output from the pump, in order to improve the accuracy of flow measurement and control. A temperature sensor 64 may be incorporated into the damper 62 or elsewhere in the circuit to enable a microprocessor 66 (discussed further hereafter) to compensate for variations in temperature of the air.

[0035] The pulsation damper 62 exhausts to atmosphere via a local flow restrictor 68, which may be an orifice plate or a porous plug of gauze or similar filter material. Alternatively it can be an adjustable valve such as a needle valve. By “local” we mean the restrictor is of only small extent in the flow direction, thereby reducing the possibility of a difficult-to-clear blockage.

[0036] A pressure sensor 70, typically of a silicon micro-machined type, measures the pressure at the inlet to the restriction 68 and thus, since the restrictor discharges to atmosphere, the pressure drop across it. The sensor provides an analogue electrical signal representative of that pressure difference. The signal is passed through a signal conditioning circuit 72 and thence to an analog to digital converter 74. The digital signal is supplied to the microprocessor 66.

[0037] The microprocessor compares the digital signal with a previously-stored target value equivalent to a desired flow rate and generates a pulse width modulated (PWM) signal to the motor drive circuitry. The PWM signal is controlled within a software control loop such that the motor speed is controlled to maintain a constant signal from the pressure sensor 70 and hence a constant flow rate.

[0038] The initial values relating pressure to flow rate are stored in the microprocessors memory and are established during an initial calibration routine. This initial procedure involves measuring the downstream pressure at two different flow rates and calculating all other values of pressure at the differing flow rates. The basic calibration curve fits a quadratic equation of the form:

Pressure=A*(flow)² +B*(flow)

[0039] where A and B are coefficients obtained from the two initial calibration points.

[0040] These calculated values of pressure for given flow are approximate. As operating the standards for the use of personal air sampling systems require routine calibration of the system using a flow tube or bubble meter, the calculated value at any particular calibration point may be overwritten by exact values.

[0041] When an adjustable valve is employed as the local restriction 68, the load on the pump can be minimised. For high flow rates, the valve is opened more than for low flow rates. Thereby the back pressure on the pump, and thus its power requirement is reduced. The microprocessor is recalibrated when the valve 70 is adjusted, so that it operates with the appropriate constants A and B in the quadratic calibration curve.

[0042] Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features.

[0043] Statements in this specification of the “objects of the invention” relate to preferred embodiments of the invention, but not necessarily to all embodiments of the invention falling within the claims.

[0044] The text of the abstract filed herewith is repeated here as part of the specification.

[0045] An improved personal sampling pump design makes use of non-circular diaphragms within the pump assembly to provide a compact, space saving design. Both upstream and downstream dampers are employed to reduce pulsation in the air flow. Flow control is achieved by monitoring the back pressure created by a flow restriction or adjustable valve located at the sampler's outlet. The created pressure is monitored by a pressure sensor and is directly related to inlet flow rate. The use of an adjustable restriction on the outlet provides increased dynamic range of the flow measurement system without excessive loading to the pump.

[0046] A signal conditioning circuit takes the signal from the pressure sensor and feeds it to an analogue to digital converter which in turn feeds it to a microprocessor. The microprocessor controls the pump drive circuitry such that a constant inlet flow rate is maintained. 

1. A displacement pump for an air sampling system comprising a pumping chamber having a displaceable wall, a motor and a drive operated by the motor for reciprocating the wall, the wall having a major dimension and a minor dimension, and being disposed with its major dimension substantially parallel to a major dimension of the motor.
 2. A pump as claimed in claim 1 wherein the major dimension of the motor is parallel to an axis of rotation thereof.
 3. A pump as claimed in claim 1 wherein there are two pumping chambers, each with a displaceable wall disposed on opposite sides of the motor.
 4. A pump as claimed in claim 1 wherein the drive comprises an eccentric mounted on a shaft of the motor.
 5. A pump as claimed in claim 1 where the wall has substantially semicircular ends joined by straight sides.
 6. A pump as claimed in claim 1 wherein the wall is a diaphragm.
 7. A displacement pump comprising a pumping chamber having a displaceable diaphragm, a motor and a drive operated by the motor for reciprocating the diaphragm, the diaphragm having a major dimension and a minor dimension, and being disposed with its major dimension substantially parallel to a major dimension of the motor.
 8. An air sampling system comprising a pump for drawing air through a sampling device, a local flow restriction downstream of the pump, a transducer for measuring a pressure drop across the restriction and for producing a signal indicative thereof, and a controller containing a correlation between the pressure drop and the volumetric flow rate of air through the restriction, the controller being configured to control the pump to produce a said pressure drop corresponding to a desired flow rate.
 9. A system as claimed in claim 8 wherein the local flow restriction is adjustable.
 10. A system as claimed in claim 9 wherein the local flow restriction is a needle valve.
 11. A system as claimed in claim 8 comprising a temperature sensor for sensing the temperature of the air passing through the restriction, the controller adjusting the said correlation according to the sensed temperature.
 12. A system as claimed in claim 8 comprising a pulsation damper downstream of the pump.
 13. A system as claimed in claim 8 comprising a pulsation damper upstream of the pump.
 14. A system as claimed in claim 11 comprising a pulsation damper downstream of the pump, the temperature sensor being located in the pulsation damper.
 15. A system as claimed in claim 8, the pump comprises a pumping chamber having a displaceable wall, a motor and a drive operated by the motor for reciprocating the wall, the wall having a major dimension and a minor dimension, and being disposed with its major dimension substantially parallel to a major dimension of the motor. 